The innervation of the pulp-dentin complex is of major health importance because it mediates dental pain and serves a poorly understood function in maintaining the vitality of teeth. Mammalian teeth are intriguing targets of sensory innervation because of their late (postnatal) development and because they have a rich nociceptive innervation confined within a rigid pulp chamber. The long-term objectives of this research program are to define the extent to which the dental pulp regulates the development, maintenance, and repair of its innervation and to elucidate the mechanisms for this regulation. The underlying hypothesis is that nerve growth factor (NGF) or other neurotrophins act as mediators by which dental tissues influence the neurons that innervate them. Work in this laboratory has demonstrated that a large proportion of the dental innervation fails to develop in the presence of anti-NGF antiserum. The specific aims of the proposed studies are to (1) determine what populations of pulpal neurons are NGF dependent; (2) characterize the NGF dependence of dental innervation development with respect to timing of the susceptibility to NGF deprivation and the mode by which innervation is lost as a result of NGF deprivation; (3) identify which neurotrophin receptors are expressed by pulpal afferent neurons and to examine the expression of these receptors in response to exposure to anti-NGF; (4) determine whether NGF is an essential mediator of the local sprouting of pulpal afferent nerves in response to tooth injury. Trigeminal neurons that project to the molar pulp in the rat will be identified by retrograde tracing, and these cells will be characterized anatomically, histochemically, and immunohistochemically. Characterization of the numbers and features of neurons persisting after anti-NGF treatment will identify which neuronal subpopulations are NGF dependent. The timing of the antiserum treatment and the timing of anatomical assessment will be varied to define the critical period of NGF dependence and the permanence of the innervation deficit that follows early NGF deprivation. The effects of NGF deprivation on both spinal and trigeminal cutaneous nerves will be assessed in parallel with the dental innervation in order to distinguish whether responses of pulpal afferents are unique. The possible role of other neurotrophins in regulation of dental innervation will be investigated by in situ hybridization analysis for neurotrophin receptor gene expression by trigeminal neurons that project to the dental pulp. The localized sprouting response, characteristic of pulpal afferent nerve fibers in response to dentinal injury, will be evaluated immunohistochemically to determine whether it is dependent on NGF availability.